In view of increasingly stringent regulation of emissions, there is an increasing need for better closed loop control of pollutants emitted in exhaust produced by operation in an engine, and for improvements in on board diagnostics (OBD) assessing the level of the emissions. One pollutant is NOx, and many engine systems utilize a NOx sensor mounted in the exhaust system to measure the amount of NOx concentration in the exhaust, and communicate the amount of NOx to the control system of the engine system. During the useful life of a typical NOx sensor, gradual fouling of the sensor's catalytic material, as well as other conditions associated with increasing age of the sensor, may cause a decrease in the accuracy of the measurements made by the sensor, and so the actual NOx concentration in the exhaust is different from the NOx concentration reported by the sensor. Systems and methods have been developed for compensating for the change in sensitivity of the NOx sensor based on age of the sensor. However, improvements are still needed in the development of compensation factors that reduce bias resulting from aging of sensors.